For example:
Given a binary tree {1,2,3,4,5},
1
/ \
2 3
/ \
4 5
return the root of the binary tree [4,5,2,#,#,3,1].
4
/ \
5 2
/ \
3 1
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public TreeNode upsideDownBinaryTree(TreeNode root) { if (root == null) return null; TreeNode left = null, right = null; TreeNode nextRoot = root.left, nextLeft = root.right; while (root != null) { root.left = left; root.right = right; left = nextLeft; right = root; root = nextRoot; if (root == null) break; nextRoot = root.left; nextLeft = root.right; } return right; } }